Vilela BG. Effect of chemomechanical procedures on bacterial activity in root canals associated with asymptomatic apical periodontitis: molecular study based on RNA and DNA [dissertation]. São Paulo: Universidade de São Paulo, Faculdade de Odontologia; 2020. Versão Corrigida.Chemomechanical procedures (CMP) represent the main strategy for root canal disinfection. However, despite substantial bacterial reduction, many canals remain infected after these procedures. As active cells present a higher abundance of ribosomal RNA (rRNA) than rDNA (rRNA genes), data obtained with rDNA-based quantitative polymerase chain reaction (qPCR) and rRNA-based qPCR (RT-qPCR) were correlated to search for active bacteria after chemomechanical procedures (CMP). Additionally, the ability of both assays to detect bacteria in endodontic samples was evaluated. Root canal samples were taken from 40 teeth with primary endodontic infections before (S1) and after CMP (S2). DNA and cDNA (synthetized from RNA) were used as templates for qPCR using universal primers for Bacteria domain. After CMP, there was a drastic reduction in the number of total bacteria. The concentration of rRNA copies in S2 samples was significantly higher than the corresponding levels of rDNA (p < 0.05), indicating the persistence of active bacteria after CMP. The rDNAbased qPCR presented low sensitivity and high specificity when compared to RT-qPCR. All samples positive for rDNA were also positive for rRNA (positive predictive value = 100%). The combined qPCR and RT-qPCR approach revealed that bacteria persisted active after CMP. Although less sensitive than RT-qPCR, the rDNA-based qPCR assays had a low risk of providing false-positive results in the analysis of total bacteria in postinstrumentation samples.